1. Field of the Invention
The present invention relates to a developing device for developing a latent image on a latent image carrier, an image forming apparatus including the developing device, and a developing method for developing the latent image.
2. Description of the Related Art
In recent years, as the printing speed becomes faster, the surface of a latent image carrier such as a photosensitive element tends to move at a higher speed. In such a high-speed image forming apparatus, unless the surface of a developer carrier moves at a high speed, the insufficient amount of toner is supplied per unit time to a developing region where the latent image carrier faces the developer carrier. As a result, a developing density deficiency may occur. For example, when a developing sleeve formed of a rotatable non-magnetic pipe is used as the developer carrier, unless the developing sleeve rotates at a high speed so that the surface thereof could move at a high speed, the insufficient amount of toner is supplied to the developing region. However, when the developing sleeve rotates at a high speed, the developer on the developing sleeve promotes the action of scraping the toner off the latent image carrier. As a result, the reproducibility of narrow-line images deteriorates.
In view of this problem, an image forming apparatus is known in which the latent image on a latent image carrier is developed by a plurality of developing sleeves. In the image forming apparatus of this type, a toner image obtained by a first developing process using a first developing sleeve is further developed by a second developing process using a second developing sleeve. A developing bias in which an alternating-current component formed by rectangular waves is superimposed on a direct-current component is applied to each of the first and second developing sleeves. In such a configuration, even when the toner image obtained by the first developing process has a developing density deficiency caused by an insufficient amount of the supplied toner, since the toner image is further developed by the second developing process, the toner image's density may be increased. In addition, in each developing process, compared to the case of applying a developing bias including only a direct-current component, in the case of applying a developing bias including both an alternating-current component and a direct-current component, the toner in the developer is more likely to move toward the latent image. In this way, developing efficiency is improved. As a result, the developing density deficiency may be suppressed without rotating the developing sleeve at a high speed.
As shown above, in a method of developing using a plurality of developing sleeves, it is a common practice to move the surface of the developing sleeve in the developing region in the same direction as the surface of the latent image carrier and to set the linear speed of the developing sleeve to be faster than the linear speed of the latent image carrier. This is based on the reasons described below. That is, when the surface of the developing sleeve in the developing region moves in the direction reverse to a moving direction of to the surface of the latent image carrier, the latent image carrier slightly brushes the developer rotating with the developing sleeve, thereby exerting a force, to the developer, which tends to move the developer in a direction reverse to the rotating direction of the developer. As a result, the force prevents the developer from rotating with the developing sleeve so that the developer stays in the developing region for a long period of time, which causes developing defects. Thus, the surface of the developing sleeve and the latent image carrier are moved in the same direction. However, if both are moved at the same linear speed in the same direction, adding the developer to the latent image on the latent image carrier may not substantially occur. Thus, the linear speed of the developing sleeve is set to be faster than the linear speed of the latent image carrier. In this way, in the developing region, the moving latent image is sequentially supplied with a new developer outrunning the latent image so that a large amount of toner can be supplied to the latent image.
However, in the above configuration, a phenomenon called a trailing blur is likely to occur in which the image density at the trailing end of a toner image becomes deficient. Especially, when a relatively large non-image portion such as a region corresponding to the gap between adjacent sheets is present on the surface of the latent image carrier, while the non-image portion passes through the developing region, a non-developing potential which elastically transfer the toner from the latent image carrier to the developing sleeve is exerted on the developing sleeve for a long period of time. As a result, most toner in the developer carried on the developing sleeve separates from the surface of the carrier particles and moves to the surface of the sleeve (hereinafter, this state will be referred to as a toner separation state). In the toner separation state, such a developer will greatly deteriorates in developing capability as compared to a normal state where most toner is present in the carrier particles. Immediately after the leading end of a latent image disposed adjacent to a relatively large non-image portion in the latent image carrier enters an entrance of the developing region along with the surface movement of the latent image carrier, the developer on the developing sleeve outrunning the leading end of the latent image enters into the toner separation state. Thus, developing defects are likely to occur. However, after that, the leading end of the latent image having moved to the vicinity of an exit of the developing region is supplied with a developer in a normal state in which toner is drawn back to the carrier particles from the surface of the sleeve as the toner passes through a portion facing the central portion of the latent image positioned closer to the rear side than the leading end of the latent image. Therefore, the leading end of the latent image is subjected to an effective developing process in the vicinity of the exit of the developing region. Such developing occurs in a developing region where the first developing sleeve faces the latent image carrier and in a developing region where the second developing sleeve faces the latent image carrier. In contrast, the trailing end of a latent image is not subjected to an effective developing process when the trailing end passes between the vicinity of the entrance of the developing region and the vicinity of the exit thereof. Specifically, immediately after the trailing end of the latent image enters the entrance of the developing region, the trail end of the latent image is supplied with a developer in the toner separation state due to the same reasons as the leading end of the latent image. After that, the trailing end of the latent image having moved to the vicinity of the exit of the developing region is supplied with only a developer which is in the toner separation state because the developer has passed through a region corresponding to a non-image portion disposed on the rear side of the trailing end of the latent image. Thus, the trailing end of the latent image is not subjected to an effective developing process even in the vicinity of the exit. As a result, a trailing blur of a toner image is likely to occur. (Japanese Patent Application Laid-open No. 2000-172064)
Therefore, there is a need for a developing device, an image forming apparatus, and a developing method capable of satisfactorily reproducing narrow-line images, suppressing the occurrence of developing flaws attributable to a developer staying in a developing region for a long period, and suppressing a trailing blur of a toner image.